1. Field of the Invention
The present invention relates to a clamping apparatus adapted to clamp an object to be clamped or fixed (referred as to a clamped object hereinafter) such as a metal mould, a work pallet and the like onto a fixed angular table of a processing machine such as an injection moulding machine, a machining center and so on, and more specifically to a fluid clamping apparatus of the type having a clamping lock means, which clamping lock means is adapted to operate so as to prevent the clamped object from being unclamped by an external force when a clamping fluid pressure abnormally lowers.
2. Description of Prior Art
Such a clamping apparatus is described in Japanese Patent Laid Open Publication No. 1 - 154833. As shown in FIG. 12, the clamping apparatus has a following basic construction.
That is, a first cylinder 309 for clamping is provided with a first piston 312 and a first actuation chamber 313. The first piston 312 is adapted to be advanced from an unclamping position Y (a figure depicted by an alternate long and two short dashes line) to a clamping position X (a figure depicted by a solid line) by means of a fluid pressure within the first actuation chamber 313. A lock chamber 326 is communicated crosswise with the first actuation chamber 313, and a wedge-type locking member 327 is inserted into the lock chamber 326. The locking member 327 is adapted to be changed over between a locking position M (a figure depicted by a solid line) where it has been advanced into the first actuation chamber 313 and a unlocking position N (a figure depicted by an alternate long and two short dashes line) where it has been retracted into the lock chamber 326. A second cylinder 329 for unlocking is provided with a second piston 339 and a second actuation chamber 340. The locking member 327 is adapted to be actuated from the locking position M to the unlocking position N by means of a fluid pressure within the second actuation chamber 340 through the second piston 339.
In the above-mentioned basic construction, a mechanism for changing over the locking member 327 is constructed as follows, as shown in FIG. 12.
That is, the second actuation chamber 340 is formed below the second piston 339, and a locking actuation chamber 393 is formed above the piston 339. A fluid pressure supply/discharge port 394 is communicated with the locking actuation chamber 393, and a locking push spring 395 is mounted in the locking actuation chamber 393.
Then, when a clamping arm 307 is changed over from an unclamping condition (a figure depicted by an alternate long and two short dashes line) to a clamping condition (a figure depicted by a solid line), a pressurized fluid is discharged from a second supply/discharge port 341 of the second actuation chamber 340 and is supplied to a first supply/discharge port 315 and to the fluid pressure supply/discharge port 394 for locking. Thereupon, by means of a fluid pressure within the first actuation chamber 313 the first piston 312 is advanced leftwards from the unclamping position Y, so that the clamping arm 307 is swung toward the clamped object 302. At the same time, by means of the fluid pressure within the locking actuation chamber 393 and a resilient force of the locking push spring 395 the locking member 327 is advanced from the unlocking position N to the locking position M, so that its wedge surface 348 engages wedgewise with a wedge receiving surface 349 of the first piston 312. By means of a resultant force of a wedge engagement force of the locking member 327 continuously advanced and of the fluid pressure within the first actuation chamber 313 the first piston 312 is pushed strongly to the clamping position X.
On the other hand, in a hydraulic clamp of the aforementioned wedge engagement type, at the end of clamping actuation the locking member 327 is actuated for locking to the locking position M against a dynamical friction force acting between both the wedge surface 348 and the wedge receiving surface 349. To the contrary, at the beginning of unclamping actuation it is necessary to actuate the locking member 327 for unlocking to the unlocking position N against a statical friction force which is remarkably larger than the aforementioned dynamical friction force. The reason for that is thought to be that the statical friction force is remarkably larger than the dynamical friction force as well as a lubricating oil is squeezed out of a gap between both surfaces 348, 349 by an excessively large surface pressure generated therebetween at the end of clamping actuation so that a metal contact is brought about therebetween.
In the above-mentioned conventional embodiment, since the resultant force of the fluid pressure within the locking actuation chamber 393 and of the resilient force of the locking push spring 395 is used to actuate the locking member 327 for locking, the locking push force is large. Therefore, when the locking member 327 is surely actuated for unlocking, it is necessary to enlarge a sectional area of the second actuation chamber 340 and provide the second piston 339 of a large diameter. Further, it is necessary to set an inclination angle .alpha. of the wedge surface 348 at a value as small as possible for surely actuating the locking member 327 for unlocking. Accordingly, the lower portion of the first piston 312 projects rightwards.
As mentioned above, since the diameter of the second piston 339 becomes large and also the length of the first piston 312 becomes long, the hydraulic clamp becomes large in size.
Further, within the duration from the initiation of the clamping actuation indicated by the figure depicted by the alternate long and two short dashes line in FIG. 12 to the starting of the wedge engagement, the lower end surface of the locking member 327 is brought into strong contact with the sealing slide surface (the outer surface) of the first piston 312 by means of the resultant force of the fluid pressure within the locking actuation chamber 393 and of the resilient force of the locking push spring 395. Therefore, the sealing slide surface gets damaged tending to cause a fluid leakage from the first actuation chamber 313. This problem appears as a serious abuse when the thickness of the clamped object such as a metal mould 302 and the like becomes larger. That is, the reason is because a load of the first piston 312 becomes large at the stage of a small swing angle of the clamping arm 307 when the thickness to be clamped becomes large and also a degree of an interference between the sealing slide surface of the first piston 312 and the lower end surface of the locking member 327 becomes large.